attempting to convert some code

[ehowington]

Wondering if someone might provide some thoughts on converting this bit of code in c++ and the next and prev in statement kinda fubars me a bit

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#include <iostream>

using namespace std;

struct Point2D
{
    double x;
    double y;
};

Point2D compute2DPolygonCentroid(const Point2D* vertices, int vertexCount)
{
    Point2D centroid = {0, 0};
    double signedArea = 0.0;
    double x0 = 0.0; // Current vertex X
    double y0 = 0.0; // Current vertex Y
    double x1 = 0.0; // Next vertex X
    double y1 = 0.0; // Next vertex Y
    double a = 0.0;  // Partial signed area

    int lastdex = vertexCount-1;
    const Point2D* prev = &(vertices[lastdex]);
    const Point2D* next;

    // For all vertices in a loop
    for (int i=0; i<vertexCount; ++i)
    {
        next = &(vertices[i]);
        x0 = prev->x;
        y0 = prev->y;
        x1 = next->x;
        y1 = next->y;
        a = x0*y1 - x1*y0;
        signedArea += a;
        centroid.x += (x0 + x1)*a;
        centroid.y += (y0 + y1)*a;
        prev = next;
    }

    signedArea *= 0.5;
    centroid.x /= (6.0*signedArea);
    centroid.y /= (6.0*signedArea);

    return centroid;
}

int main()
{
    Point2D polygon[] = {{0.0,0.0}, {0.0,10.0}, {10.0,10.0}, {10.0,0.0}};
    size_t vertexCount = sizeof(polygon) / sizeof(polygon[0]);
    Point2D centroid = compute2DPolygonCentroid(polygon, vertexCount);
    std::cout << "Centroid is (" << centroid.x << ", " << centroid.y << ")\n";
}

[ehowington]

Please note this is based off of Emile Cormier's algorithm

[ehowington]

Far as I have gotten with this bit of code

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Structure Point2D
   x.d;
   y.d;
EndStructure

Point2D compute2DPolygonCentroid(const Point2D* vertices,vertexCount.i)
Point2D = 0
centroid = 0
signedArea.d = 0.0;
x0.d = 0.0; // Current vertex X
y0.d = 0.0; // Current vertex Y
x1.d = 0.0; // Next vertex X
y1.d = 0.0; // Next vertex Y
a.d = 0.0;  // Partial signed area

lastdex.i = vertexCount -1;
                          ;const Point2D* 
prev = &(vertices[lastdex]);
    ;const Point2D* Next;

    // For all vertices in a loop
    For i.i=0 To vertexCount Step 1
    
        
        x0 = prev->x;
        y0 = prev->y;
        x1 = Next->x;
        y1 = Next->y;
        a = x0*y1 - x1*y0;
        signedArea = a + a;
        centroid.x = (x0 + x1)*a + (x0 + x1)*a ;
        centroid.y = (y0 + y1)*a + (x0 + x1)*a;
        ;prev = Next;
    
    Next
    signedArea = 0.5 * 0.5;
    centroid.x = (6.0*signedArea)/(6.0*signedArea);
    centroid.y = (6.0*signedArea)/(6.0*signedArea);

    Return centroid;
}

int main()
{
    Point2D polygon[] = {{0.0,0.0}, {0.0,10.0}, {10.0,10.0}, {10.0,0.0}};
    size_t vertexCount = SizeOf(polygon) / SizeOf(polygon[0]);
    Point2D centroid = compute2DPolygonCentroid(polygon, vertexCount);
    std::cout << "Centroid is (" << centroid.x << ", " << centroid.y << ")\n";
}

[wilbert]

Something like this ?

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Structure Point2D
  x.d
  y.d
EndStructure

Procedure compute2DPolygonCentroid(Array vertices.Point2D(1), *centroid.Point2D)
  Protected.Point2D centroid, *prev, *next
  Protected.d signedArea, x0, y0, x1, y1, a
  Protected.i i, lastdex
  
  lastdex = ArraySize(vertices())
  *prev = @vertices(lastdex)
  
  For i = 0 To lastdex
    *next = @vertices(i)
    x0 = *prev\x
    y0 = *prev\y
    x1 = *next\x
    y1 = *next\y
    a = x0*y1 - x1*y0
    signedArea + a
    centroid\x + (x0 + x1)*a
    centroid\y + (y0 + y1)*a
    *prev = *next
  Next
  
  signedArea * 0.5
  *centroid\x = centroid\x / (6.0*signedArea)
  *centroid\y = centroid\y / (6.0*signedArea)
EndProcedure


Dim polygon.Point2D(3)
polygon(0)\x = 0
polygon(0)\y = 0
polygon(1)\x = 0
polygon(1)\y = 10
polygon(2)\x = 10
polygon(2)\y = 10
polygon(3)\x = 10
polygon(3)\y = 0

compute2DPolygonCentroid(polygon(), @centroid.Point2D)
Debug "Centroid is (" + centroid\x + ", " + centroid\y + ")"

[ehowington]

hmmm not exactly

Code: Select all

Structure Point2D
  x.d
  y.d
EndStructure

Procedure compute2DPolygonCentroid(Array vertices.Point2D(1), *centroid.Point2D)
  Protected.Point2D centroid, *prev, *next
  Protected.d signedArea, x0, y0, x1, y1, a
  Protected.i i, lastdex
  
  lastdex = ArraySize(vertices())
  *prev = @vertices(lastdex)
  
  For i = 0 To lastdex
    *next = @vertices(i)
    x0 = *prev\x
    y0 = *prev\y
    x1 = *next\x
    y1 = *next\y
    a = x0*y1 - x1*y0
    signedArea + a
    centroid\x + (x0 + x1)*a
    centroid\y + (y0 + y1)*a
    *prev = *next
  Next
  
  signedArea * 0.5
  *centroid\x = centroid\x / (6.0*signedArea)
  *centroid\y = centroid\y / (6.0*signedArea)
EndProcedure


Dim polygon.Point2D(3)
polygon(0)\x = 0
polygon(0)\y = 0

polygon(1)\x = 20
polygon(1)\y = 0

polygon(2)\x = 20
polygon(2)\y = 20

polygon(3)\x = 10
polygon(3)\y = 30

polygon(4)\x = 0
polygon(4)\y = 20

compute2DPolygonCentroid(polygon(), @centroid.Point2D)
Debug "Centroid is (" + centroid\x + ", " + centroid\y + ")"

[ehowington]

also even closing of polygon

Code: Select all

Structure Point2D
  x.d
  y.d
EndStructure

Procedure compute2DPolygonCentroid(Array vertices.Point2D(1), *centroid.Point2D)
  Protected.Point2D centroid, *prev, *next
  Protected.d signedArea, x0, y0, x1, y1, a
  Protected.i i, lastdex
  
  lastdex = ArraySize(vertices())
  *prev = @vertices(lastdex)
  
  For i = 0 To lastdex
    *next = @vertices(i)
    x0 = *prev\x
    y0 = *prev\y
    x1 = *next\x
    y1 = *next\y
    a = x0*y1 - x1*y0
    signedArea + a
    centroid\x + (x0 + x1)*a
    centroid\y + (y0 + y1)*a
    *prev = *next
  Next
  
  signedArea * 0.5
  *centroid\x = centroid\x / (6.0*signedArea)
  *centroid\y = centroid\y / (6.0*signedArea)
EndProcedure


Dim polygon.Point2D(3)
polygon(0)\x = 0
polygon(0)\y = 0

polygon(1)\x = 20
polygon(1)\y = 0

polygon(2)\x = 20
polygon(2)\y = 20

polygon(3)\x = 0
polygon(3)\y = 20

polygon(4)\x = 0
polygon(4)\y = 0

compute2DPolygonCentroid(polygon(), @centroid.Point2D)
Debug "Centroid is (" + centroid\x + ", " + centroid\y + ")"

[wilbert]

You added a new point to the array but didn't change the dim statement to
Dim polygon.Point2D(4)

[ehowington]

ah woops lol I see it now

[ehowington]

Many thanks

Code: Select all

Structure Point2D
  x.d
  y.d
EndStructure

Procedure compute2DPolygonCentroid(Array vertices.Point2D(1), *centroid.Point2D)
  Protected.Point2D centroid, *prev, *next
  Protected.d signedArea, x0, y0, x1, y1, a
  Protected.i i, lastdex
  
  lastdex = ArraySize(vertices())
  *prev = @vertices(lastdex)
  
  For i = 0 To lastdex
    *next = @vertices(i)
    x0 = *prev\x
    y0 = *prev\y
    x1 = *next\x
    y1 = *next\y
    a = x0*y1 - x1*y0
    signedArea + a
    centroid\x + (x0 + x1)*a
    centroid\y + (y0 + y1)*a
    *prev = *next
  Next
  
  signedArea * 0.5
  *centroid\x = centroid\x / (6.0*signedArea)
  *centroid\y = centroid\y / (6.0*signedArea)
EndProcedure


Dim polygon.Point2D(5)
polygon(0)\x = 0
polygon(0)\y = 0

polygon(1)\x = 20
polygon(1)\y = 0

polygon(2)\x = 20
polygon(2)\y = 20

polygon(3)\x = 10
polygon(3)\y = 100

polygon(4)\x = 0
polygon(4)\y = 20

polygon(5)\x = 0
polygon(5)\y = 0

compute2DPolygonCentroid(polygon(), @centroid.Point2D)
Debug "Centroid is (" + centroid\x + ", " + centroid\y + ")"